In a small skillet/frying pan, melt the beef suet/tallow over a medium heat then add the onion. Cook, stirring regularly, until it has softened but is not browning (if it starts to brown then reduce the heat).
Reduce the heat to low, add the oatmeal and stir to mix. Season with a pinch of salt and pepper. Allow to cook, stirring now and then, for around 15 minutes until the oats are golden and cooked through. They will be a bit nutty and have some chew, but should taste cooked. If you want them slightly softer, add a teaspoon or two of stock but it shouldn't really be needed.
Preheat the oven to 375F/190C.
If needed, use the flat side of a mallet or a rolling pin to flatten out the chicken tenders a bit to make them relatively easy to roll. If using whole breasts, you will need to cut them probably in 4 or more pieces or, if serving as a main, you can just slit a pocket in the middle and fill it as a whole breast.
Put a spoonful of the oatmeal mixture onto each piece of chicken, pressing it together a little, and carefully roll them up and place in an oven dish, join down. If they don't seem to hold you can secure with a cocktail stick. Don't worry if it leaks a little, just try to stuff back in. Repeat with the rest of the pieces of chicken and stuffing. Drizzle a little oil over the top of the pieces of chicken.
Bake the chicken for approximately 20 - 30 minutes until cooked through.